1. Field of the Invention
The present invention relates to microwave assisted magnetic heads and magnetic disk devices providing a microwave radiation waveguide, which generate an alternating current (AC) magnetic field of a microwave band, to assist in writing data signals on magnetic recording media having a large coercive force to stabilize the magnetization.
2. Description of Conventional Art
In association with an advance of high density recording, bit cells for recording digital information on magnetic recording media are miniaturized. As a result, since signals detected by a reproducing element of a magnetic head fluctuate due to so-called thermal fluctuation, a signal-noise ratio (S/N) may be deteriorated or the signal may be lost in the worst case.
For a magnetic recording medium of a perpendicular recording system, which is utilized in practice in recent years, an increase in perpendicular magnetic anisotropy energy Ku of a recording film is effective to solve the above-described problem. A stability thermal stability coefficient that corresponds to the thermal fluctuation is given by Ku·V/kB·T. Herein, Ku is perpendicular magnetic anisotropy energy, V is a volume of one magnetic microparticle, kB is the Boltzmann constant, and T is the absolute temperature.
According to the so-called Stoner-Wohlfarth model, anisotropy energy Hk and coercive force He of the recording film are indicated by a formula below. In accordance with the increase in Ku, the coercive force He also increases (additionally, Hk>Hc for normal recording films).Hk=Hc=2Ku/Ms 
Herein, Ms is a saturation magnetization of the recording film.
For a reversal of magnetization of the recording film corresponding to a desired data sequence, it is necessary to apply a recording magnetic field that is steep and approximately around Hk at maximum. For magnetic disk devices (or hard disk drive, HDD), which are utilized in practice in recent years because of the perpendicular recording system, a recording element with a so-called single magnetic pole is utilized. A recording magnetic field is applied, which is perpendicular to a recording film from a surface of an air bearing surface (ABS).
An intensity of a perpendicular recording magnetic field is proportional to a saturation magnetic flux density Bs of a soft magnetic material forming the single magnetic pole. Therefore, materials having a saturation magnetic flux density Bs as high as possible are developed and utilized in practice.
However, according to the so-called Slater-Pauling curve, Bs-2.4 T (tesla) is a limit of the saturation magnetic flux density Bs for practical use, and currently it is approaching the limit for practical use.
A thickness and/or a width of a current single magnetic pole is approximately 100-200 nm. In order to increase a recording density, further reduction of the thickness and/or width is required, and the perpendicular magnetic field generated with such a minute magnetic pole tends to be reduced.
For these reasons, it can be said that the recording ability of the ordinary data writing element is approaching the limit, and that difficulties are faced to achieve the high density recording.
Therefore, a so-called thermal assisted magnetic recording (TAMR) has been proposed. With the TAMR, the recording film is irradiated with laser light etc., the temperature of the recording film is increased, and signals are recorded in a situation where the coercive force of the recording film is lowered.
However, there are the following problems even for the TAMR. (1) A magnetic head providing a magnetic element and an optical element is required so that the configuration thereof is extremely complex and expensive. (2) It is required to develop a recording film which has a coercive force with a highly sensitive temperature characteristic. (3) Due to a thermal demagnetization during a recording process, adjacent track erasures may occur and/or a recording condition becomes unstable.
On the other hand, in order to largely reduce perpendicular recording magnetic fields that are necessary for magnetization reversal, it is considered to overlap AC magnetic fields in an in-plane direction with a microwave band on a perpendicular recording magnetic field generated from a tip of a main pole for exciting the magnetization reversal. The AC magnetic fields are the same as, or close to, a ferromagnetic resonant frequency of a medium recording layer. Such an assisted recording method is referred to as microwave assisted magnetic recording (MAMR), and its efficiencies are experimentally verified.
With respect to the MAMR, two methods have been mainly proposed. One is a method that generates a microwave magnetic field in the in-plane direction by forming a spin torque oscillator (STO) formed of a multilayered magnetic thin film in a gap (write gap) between a main pole (or write pole) of the magnetic head and an auxiliary magnetic pole that is a write shield, and by driving a bias electric current to oscillate the STO, as discussed in Reference 1 (J. Zhu et al.; IEEE Transaction on Magnetics, Vol. 44, No. 1, p. 125) (this may be called a STO type).
The other is a method that generates an in-plane AC magnetic field by providing a secondary coil in, or adjacent to, the write gap between the main pole and the auxiliary magnetic pole of the magnetic head and by driving an AC of a microwave band to the secondary coil, as discussed in Reference 2 (JP Patent Laid-open Publication 2007-299460) (this may be called a coplanar waveguide (CPW) type).
The STO type has a complex process because an STO element configured of multilayered films is embedded in the write gap that is in a scale of approximately 30 nm, and an oscillation frequency and power of the type has a limit due to a configuration of the STO element and an applied bias. Therefore, it is assumed that the STO type lacks versatility for all types of the perpendicular magnetic recording medium.
With the above described CPW type, which is different from the STO type, its frequency and power are arbitrarily set by a high frequency oscillation source mounted outside. However, it is required to form a coil conductor in the write gap and to embed a periphery thereof with insulators, so that there are structural and dimensional limitations and the process is complex.
The present invention is conceived corresponding to the current situation. One of objectives of the present invention is to provide a microwave assisted magnetic head that has a novel configuration, having a simple configuration, with a relatively easy and efficient manufacturing process, and that overlaps AC magnetic fields in an in-plane direction of a microwave band, which is the same as, or close to, a ferromagnetic resonant frequency of a medium recording layer.